A wind turbine blade cross section is typically referred to as a profile. The blade is connected to the hub that is placed in the rotor centre. The profile has a chord, c, and a thickness, t, as shown in FIG. 1. The shape of the profile, e.g. the sizes of the chord and the thickness as well as the thickness to chord ratio, varies as a function of the radius, r, i.e. the distance from the rotor centre to the blade cross section.
Typically, a wind turbine blade airfoil is formed by interpolation between a plurality of profiles. The blade and hence the individual profiles are rotated relative to the rotor plane during operation. The incoming wind is about orthogonal to the rotor plane, but since the blade is in motion, the effective angle and speed of the incoming wind (i.e. corresponding to a steady blade) depend on the speed of rotation of the blade. The effective angle is also referred to as the angle of attack, α, as shown in FIG. 2. The effective wind speed that the profiles experience is also referred to as the relative wind speed, w, as shown in FIG. 2.
A wind turbine blade may be prone to both permanent and temporary surface irregularities during the service life. Temporary surface irregularities may e.g. be bird droppings, insects, dust particles, rain, snow, ice, salt, etc. Permanent irregularities may e.g. originate from temporary surface irregularities, which are not removed, or arise during manufacturing or handling, e.g. via scratches, mould imperfections, paint defects, etc.
Presence of surface irregularities typically results in one or more of reduced lift, increased drag and increased acoustic emission, which changes all are undesired for the application of blades for a modern wind turbine.
A certain number and size of surface irregularities are unavoidable through the service life of a wind turbine blade. There is hence a demand for a wind turbine blade and a profile, for which the aerodynamic properties are less influenced by surface irregularities.